Hello, yes, I've seen dirt/debris build up under the actuators for the drive control levers that restricts full movement and engagement of the drive controls, but if your machine suffers from slow/weak drive in both directions equally, it's likely not a control valve problem. Likewise, if both left and right drives seem equally weak, in other words, one side doesn't drive faster than the other, then it's likely not a drive motor problem. I only say that because the chances are slim that both drive motors crap out at the exact same time. It's not impossible, but improbable.
The best way to do a test of the drive motors is to remove the case drain line from the motor (be sure and cap the hose that you removed or it will leak oil from tank), then raise that track off the ground with the boom and operate that drive while you monitor leakage (flow) from the case drain port on the motor. If it leaks out more than a gallon of oil in a minute then the motor is suspect inside.
One other thing I can say about those machines is your problem can be in the rotary joint that delivers oil from the upper structure to the lower track frame. I've replace the rotary joint seals in several E70B and E120B Cat machines that was doing the same thing your machine is doing, weak drives, you could hardly get them to turn with the tracks. You might consider dropping that rotary joint and fitting new seals in it first. For someone who's mechanically inclined it's not a terribly difficult task, and even if it turns out to not be the problem, it's a major component that's been refreshed for not a lot of coin. Also, even if it's not the problem at present, it would be in the future because those seals will eventually fail on that vintage machine.